1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method of manufacturing a solid electrolytic capacitor, and particularly to a solid electrolytic capacitor having a low equivalent series resistance and a method of manufacturing the solid electrolytic capacitor.
2. Description of the Related Art
Conventionally, as a solid electrolytic capacitor, a solid electrolytic capacitor having an anode portion made of a valve metal and a solid electrolyte has been known. As the anode portion, a sintered body of valve metal particles, an etched valve metal plate, and an etched valve metal foil may be used. In the following, a description will be given of a structure of the conventional solid electrolytic capacitor in which a sintered body of valve metal particles is used as an anode portion.
FIG. 6 is a schematic cross section showing an example of the structure of the conventional solid electrolytic capacitor. In FIG. 6, solid electrolytic capacitor 600 includes a capacitor element 60 having a dielectric coating 63, a solid electrolyte 64, and a cathode lead portion 65 that are formed in order on a surface of an anode portion 61 on which an anode lead portion 62 is erected. Anode lead portion 62 has an exposed end connected to an anode terminal 66 through a coupling portion 67 made of a metal, and cathode lead portion 65 is connected to a cathode terminal 68 through an adhesion layer 69. Capacitor element 60 is sealed with an outer coating resin 70.
Dielectric coating 63 may be formed by chemical conversion of anode portion 61 made of a valve metal. For solid electrolyte 64, a conductive polymer formed by chemical polymerization or electrolytic polymerization for example may be used. For cathode lead portion 65, silver or a combination of carbon and silver for example may be used.
Solid electrolytic capacitor 600 as described above has a remarkably compact and highly durable dielectric coating formed therein, and therefore can be downsized without reduction of the capacitance, as compared with other capacitors such as paper capacitor and film capacitor, for example. Further, the conductive polymer has a high electrical conductivity, and the equivalent series resistance (hereinafter “ESR”) of the solid electrolytic capacitor can be reduced.
As a material that forms adhesion layer 69 connecting cathode lead portion 65 and cathode terminal 68, a conductive paste is widely used (Japanese Patent Laying-Open No. 2009-218502). The conductive paste refers to a paste of a mixture of an adhesive having adhesiveness and a metal filler having electrical conductivity. The paste is applied between cathode lead portion 65 and cathode terminal 68 and thereafter heated so that the adhesive is set. In this way, cathode lead portion 65 and cathode terminal 68 can be fixed to each other, and adhesion layer 69 having electrical conductivity can be formed.
However, adhesion layer 69 contains the electrically conductive metal filler and also contains the electrically insulating adhesive, which results in a problem that adhesion layer 69 itself has a resistance. The fact that adhesion layer 69 itself has a resistance results in a problem of an increased ESR of solid electrolytic capacitor 600.
FIG. 7 shows a schematic enlarged view of a region A in FIG. 6. It is known that insulating layers 73a, 73b without metal filler 72 are formed on the surface where adhesion layer 69 contacts cathode lead portion 65 and the surface where adhesion layer 69 contacts cathode terminal 68, respectively. Such insulating layers 73a, 73b cause a contact resistance between adhesion layer 69 and cathode lead portion 65 and a contact resistance between adhesion layer 69 and cathode terminal 68, which leads to a problem of a further increased ESR of solid electrolytic capacitor 600.